In forming an electronic circuit on a metallic substrate, an insulating paste, for example, is used to form an insulating layer on the substrate first, and then an electronic circuit including a conductive layer is formed on the insulating layer.
In this case, if the insulating layer has a thermal expansion coefficient considerably different from that of the metallic substrate, the metallic substrate may suffer warpage and/or the insulating layer may suffer cracking during firing or the like. Therefore, for matching the thermal expansion coefficients of the metallic substrate and the insulating layer, a ceramic filler is generally added to the paste.
Particularly, in the case of forming an insulating layer made up of a plurality of layers by repeating application of the insulating paste and firing, the lower layers constituting the insulating layer are exposed to high temperature many times repeatedly, which may promote crystallization or cause reaction with the filler.
Patent Document 1 below discloses an invention about a crystallizable glass for use as a dielectric layer on a substrate, and a thick membranous composition of the glass.
Patent Document 2 discloses a glass composition for use on an aluminum nitride substrate and a thick film dielectric composition containing the glass composition.
Patent Document 3 discloses an insulating layer used for forming an electronic circuit on a substrate made of metal or ceramic, and a method for producing the insulating layer.